The present invention is intended to be used in a data switching node for switching data from a plurality of source data switching units to a plurality of destination data switching units connected by means of time bus connectors to a looped unidirectional time bus wherein the total information carrying capacity of said time bus is time division multiplexed into a plurality of separate data channels comprising data switching control unit means for the connection of said source and destination data switching units.
Thus, the present invention relates to loop type communication systems in general and to local data switching nodes in message and packet switching networks in particular.
In the prior art, loop type communication systems have employed several time division multiplexing techniques. One such technique permanently assigns separate data slots or channels to each device connected on the looped bus, so that each device is only able to transmit in its assigned data channel. This is inefficient as a large part of the communication bandwidth is wasted if only a few of the total number of connected devices are communicating at a given moment.
Another technique uses addressed messages, thus allowing devices to utilise any data channel, but additional communication space is required for the addresses, while the problem of devices attempting to communicate at the same time has to be resolved by polling techniques.
Still another technique uses a combination of permanently assigned data channels for each device together with dynamically assigned data channels, where a master controller assigns additional data channels to specific devices when required. This adds flexibility but the terminal must be commanded, that is, a master controller is required to control the whole communication loop.
In the context of message switching systems the high speed transmission of data on the local loop or node is not the only consideration. The desirable characteristics of a message or packet switching system necessitate that a local node must not only be able to switch messages originating from devices connected to the local node to other destination devices connected to the same node, but must also be able to accept and switch incoming messages originating on remote nodes or devices to destination devices on the local node or remote nodes. Thus, a data switching node, in addition to a high data transmission rate, also requires the following specific characteristics.
1. High reliability to ensure that data is not lost.
2. Flexibility to handle different ranges of data volume without causing congestion on the node.
3. Flexibility to expand the capacity of the node to accomodate larger message switching system traffic.
4. Interface standardisation allowing easy connection between different and often incompatible data handling equipment and terminals.
The time division multiplexing techniques mentioned have also been used for message switching systems.